1. Field of the Invention
The present invention relates to a converter for satellite broadcast reception for receiving radio waves that are transmitted from two satellites adjacent to each other. In particular, the invention relates to a converter for satellite broadcast reception that is suitable for the reception of circularly polarized radio waves that are transmitted from each satellite.
2. Description of the Related Art
In a converter for satellite broadcast reception for receiving radio waves that are transmitted from a plurality of satellites adjacent to each other, to receive, with one LNB (low noise block converter), left-handed polarized and right-handed polarized satellite broadcast signals that are transmitted from each of two satellites, for example, by causing those signals to enter separate waveguides, it is necessary to convert the left-handed polarized waves and right-handed polarized waves that have entered each waveguide into vertically polarized waves and horizontally polarized waves with a phase conversion portion and then receive the vertically polarized waves and the horizontally polarized waves by inputting those to a pair of probes.
As an example of such a converter for two-satellite broadcast reception, a converter is known in which dielectric feeders are held by the front end portions of two respective waveguides, a circuit board is disposed on the rear end side of the waveguides, and two sets of a probe for vertically polarized waves and a probe for horizontally polarized waves are patterned on the same surface of the circuit board in such a manner that the two sets correspond to the respective waveguides. A radiation portion and a phase conversion portion are integrated with each dielectric feeder at its respective ends in such a manner that the radiation portion projects forward from the open end of the waveguide and the phase conversion portion is inserted in and fixed to the waveguide. The probe for vertically polarized waves and the probe for horizontally polarized waves of each set are generally perpendicular to each other on the circuit board and the phase conversion portion of the dielectric feeder crosses each of the probe for vertically polarized waves and the probe for horizontally polarized waves so as to form an angle of about 45xc2x0. The circuit board is also provided with processing circuits, by which signals detected by the respective probes are frequency-converted into different intermediate frequency bands.
In the converter for two-satellite broadcast reception having the above-outlined configuration, when left-handed polarized waves and right-handed polarized waves that have been transmitted from each satellite enter one of the two dielectric feeders via the radiation portion, the left-handed polarized waves and the right-handed polarized waves are converted into vertically polarized waves and horizontally polarized waves in traveling through the dielectric feeder, which are input to the probe for vertically polarized waves and the probe for horizontally polarized waves that are provided on the circuit board. The use of the dielectric feeders having the phase conversion portions simplifies the shape of the waveguides thereby enables manufacturing cost reduction. And patterning the probes on the same surface shortens the overall length of the waveguides themselves and thereby makes it possible to reduce the size of the converter.
Incidentally, in the above conventional converter for satellite broadcast reception, since the two sets of a probe for vertically polarized waves and a probe for horizontally polarized waves are pattern on the same surface of the circuit board, there is a problem that isolation between vertically polarized waves and horizontally polarized waves is insufficient and hence a good cross-polarization characteristic cannot be obtained. To solve this problem, a technique has been proposed in which isolation between vertically polarized waves and horizontally polarized waves is secured by forming square or circular minute radiation patterns are formed on the circuit board at intersecting points of the extensions of the probes for vertically polarized waves and the probes for horizontally polarized waves.
However, each minute radiation pattern is symmetrical with respect to the axial lines of the probe for vertically polarized waves and the probe for horizontally polarized waves. Therefore, if the size (area) of each minute radiation pattern is made small, good isolation between vertically polarized waves and horizontally polarized waves cannot be obtained. Conversely, if each minute radiation pattern is made large, a problem arises that the reflection component increases to cause undue transmission loss. The use of such minute radiation patterns causes another problem. If the positional relationship between the dielectric feeder and the minute radiation pattern and other factors are not the same in the two waveguides, a phase deviation occurs between linearly polarized waves in either waveguide. Therefore, the layout of the probes and signal lines on the circuit board is determined automatically; that is, the degree of freedom in circuit designing is low.
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide a converter for satellite broadcast reception capable of increasing the degree of freedom in circuit designing while securing isolation between vertically polarized waves and horizontally polarized waves.
To attain the above object, the invention provides a converter for satellite broadcast reception having a pair of hollow waveguides, first and second dielectric feeders held by the respective waveguides, and a circuit board that is disposed perpendicularly to the axial lines of the respective waveguides in which left-handed and right-handed circularly polarized waves transmitted from each of two satellites adjacent to each other enter a radiation portion of one of the first and second dielectric feeders and are converted by a phase conversion portion of the one of the first and second dielectric feeders into vertically polarized waves and horizontally polarized waves, respectively, which are input to a probe for vertically polarized waves and a probe for horizontally polarized waves, respectively, that are provided on the circuit board, the converter comprising a first minute radiation pattern and a second minute radiation pattern each being provided on the circuit board so as to be inclined electrically by about 45xc2x0 from the respective axial lines of the probe for vertically polarized waves and the probe for horizontally polarized waves, the first minute radiation pattern being approximately perpendicular to the phase conversion portion of the first dielectric feeder, the second minute radiation pattern being approximately parallel with the phase conversion portion of the second dielectric feeder, wherein the phase conversion portion of the first dielectric feeder is longer than that of the second dielectric feeder.
In the above-configured converter for satellite broadcast reception, each of the first minute radiation pattern and the second minute radiation pattern that are formed on the circuit board so as to correspond to the two respective dielectric feeders is inclined electrically by about 45xc2x0 from the axial lines of the probe for vertically polarized waves and the probe for horizontally polarized waves. Therefore, the electric field disorder in each waveguide is suppressed by the relatively small, minute radiation pattern, and hence isolation between vertically polarized waves and horizontally polarized waves can be secured. Since the first minute radiation pattern is approximately perpendicular to the phase conversion portion of the first dielectric feeder and the second minute radiation pattern is approximately parallel with the phase conversion portion of the second dielectric feeder, the degree of freedom in the layout of the probes and signal lines on the circuit board is increased. Further, since the one phase conversion portion that is approximately perpendicular to the minute radiation pattern is longer than the other phase conversion portion that is approximately parallel with the minute radiation pattern, a phase deviation that is caused by the difference in the angle between the phase conversion portion and the minute radiation pattern can be corrected for, whereby satellite broadcast signals transmitted from the two satellites can be received reliably.
In the above configuration, it is preferable that one of a first pair of signal lines that are connected to the respective probes for vertically polarized waves and a second pair of signal lines that are connected to the respective probes for horizontally polarized waves be disposed close to the center of the circuit board and the other be disposed outside the one pair. This makes it possible to frequency-convert left-handed circularly polarized signals and right-handed circularly polarized signals from the two satellites into signals in different intermediate frequency bands by using common oscillators, and to thereby simplify the circuit configuration.
In the above configuration, each of the first dielectric feeder and the second dielectric feeder may be an integral mold member. However, it is preferable that each of the first and second dielectric feeders be composed of a first divisional body having the radiation portion and a second divisional body having the phase conversion portion and the first and second divisional bodies be integrated with each other by inserting a projection that is provided in the second divisional body into a through-hole that is formed in the first divisional body. Dividing each dielectric feeder into the first and second divisional bodies in this manner makes the volume (capacity) of each of the first and second divisional bodies small, and the probability of occurrence of a sink or air bubble can be lowered accordingly. Further, since each dielectric feeder is divided at the portion where the projection is joined to the surface of the through-hole and the dividing surface is distant from the center of the first divisional body where the electric field is strongest, adverse electrical effects due to the division can be made small.
In this case, it is preferable that the second divisional body have an impedance conversion portion that assumes arcs in cross section that become closer to each other as the position goes away from the open end of the waveguide toward the phase conversion portion, the projection project from an end face of the impedance conversion portion, and the first and second divisional bodies be joined to each other at the end face of the impedance conversion portion. With such an impedance conversion portion, the reflection component of radio waves that travel from the radiation portion to the phase conversion portion past the impedance conversion portion can be weakened to a large extent. Further, a large phase difference is obtained for linearly polarized waves even if the length of the portion from the impedance conversion portion to the phase conversion portion is reduced, which makes it possible to greatly reduce the total length of the waveguide.
In the above configuration, at least one of the two respective second divisional bodies of the first and second dielectric feeders may be provided with an identification mark that allows the two second divisional bodies to be discriminated from each other visually. This allows each first divisional body to be held by the corresponding waveguide reliably without causing erroneous insertion. In this case, the second divisional bodies having different lengths may be molded so as to assume different colors. This requires merely coloring an injection molding material and hence can lower manufacturing cost increase.